an energetic, sometimes boisterous girl whose behavior and pursuits, especially in games and sports, are considered more typical of boys than of girls.
1550s, "rude, boisterous boy," from Tom + boy; meaning "bold or immodest woman" is attested from 1570s; that of "girl who acts like a spirited boy" is first recorded 1590s.
Slang definitions & phrases for tomboy
tomboy
noun
A young girl who prefers boyish pursuits, looks, etc, to the presumed feminine ones (1592+)